[Craxuan]

The tax is used to pay for the Army's equipments, rations, etc. But for the soldiers Kirito and Asuna met, that is just plain bully.

Spoiler for Army Org. Objective Spoiler:

The Army is originally an organization built to feed all the players in SAO and to ensure everyone's survival. Those who fight will gather whatever resources they can gather and distribute them equally to everyone. Two years later, that objective becomes distorted because of its immense size and it became a form of tax instead, where the non-fighters are forced to pay up their resources to the front liners for better equipment. The practice was originally just a donation by those who feel grateful for those who fought for them at the front lines, but again it became distorted and became plain extortion.

Also, people who think you can't get hurt just because you're in a safe zone is wrong. You have seen firsthand - not as severe as actually described in the novel - how Asuna beat the crap out of the soldier without consequences. All it does a 'mild knockback effect' when you sword skill someone to the face. Now imagine that happen for a prolonged amount of time, let's say five minutes straight, where you just bounce all around the place as the torturer repeatedly beats you with sword skills. Even if you don't feel pain, just the dizziness is enough to scare a person's pants off.

And what Asuna did was just an example how you can abuse the absolute safety within the Safe Zone. There are plenty of ways - each and everyone of them far more painful than being beaten physically - to toy with a person's heart without putting a scratch onto someone.